The invention relates generally to resonator cavity designs for solid-state lasers and more particularly to resonator cavity designs for diode-pumped solid-state lasers and to diode-pumped solid-state laser amplifiers.
Conventional optically-pumped solid-state lasers utilize broadband arc lamps or flashlamps to laterally or transversely pump the solid-state laser medium in a resonant cavity. The direction of pumping is transverse or orthogonal to the longitudinal axis of the resonant cavity. The entire medium is pumped so there is little correspondence between the pump volume and the TEMOO mode volume defined by the laser cavity; operation in TEMOO mode is desired. Much of the pumping energy goes into regions of the medium outside the volume occupied by the laser mode and therefore does not contribute to amplification of the laser beam. Thus pumping efficiency is low (typically a few percent).
Laser diodes form efficient pumping sources; a variety of different types of laser diodes, particularly laser diode arrays, e.g. Spectra Diode Labs Model No. 2410 GaAlAs laser diode array, in which a plurality of emitters are phase locked together, and extended emitter laser diodes, e.g. Sony Model Nos. SLD 301, 302, 303, 304 V/W, have been or can be used. U.S. Pat. Nos. 4,653,056 and 4,656,635 and patent applications Ser. Nos. 029,836 filed Mar. 24, 1987 and 035,530, filed Apr. 7, 1987 describe a solid-state laser longitudinally end pumped by a laser diode source in which the pump volume is matched to the desired TEMOO mode volume to optimize pumping efficiency. In the longitudinal end pump configuration, the direction of pumping coincides with the longitudinal axis of the resonator cavity, and thus can be matched into the laser mode volume. U.S. Pat. No. 4,665,529, issued May 12, 1987 and patent application Ser. No. 048,717 filed May 12, 1987 describe a solid-state laser in which a laser diode source is coupled to a laser rod by means of an optical fiber to longitudinally end pump and mode match the laser. It is desirable to produce small size, low cost, high performance solid-state lasers.
Thus the resonator/pump configuration is a key feature of laser design and performance. Lateral pumping schemes do not provide mode matching and are therefore inefficient. End pumping schemes using laser diodes provide mode matching and consequently high efficiency. However, previously-available laser diodes have often been limited in power, usually under 1 W Furthermore, even with higher power laser diode sources, the end pumped configuration limits the amount of energy that can be used, thereby limiting the power of the laser, since the power densities in the pump region of the gain medium become too high and the heat produced cannot be removed. This heat is disadvantageos in that it causes thermal focusing and bulk heating leading to general inefficiency. Accordingly, it is desirable to provide a resonator configuration which combines a transverse or lateral pump geometry with mode matching of the pump volume to the TEMOO mode volume since lateral pumping allows more energy to be input into the medium while mode matching uses the pump energy more effectively.
Another type of laser diode is a plurality of laser diode arrays fabricated into a multi-element bar structure. These laser diode array bars typically have ten 1 W laser diode arrays spaced along a 1 cm bar; each array has multiple emitters phase locked together. These array bars are not suitable for end pumping a solid-state laser but could be useful for transversely or laterally pumping a solid-state laser. However, if the bars are used as mere substitutes for arc lamps, little benefit will be derived. Accordingly it is necessary to develop a laser resonator/pump configuration in which the output of the laser diode array bar can be mode-matched to a desired mode volume (TEMOO) within the solid-state laser material.
It is sometimes desireable to obtain laser power outputs greater than 10 watts. However, at these power levels, at least two problems occur which prevent realization of larger power outputs. First, pump power must be increased. As the pump power increases, the laser rod heats up, thus reducing laser efficiency. Cooling of the rod is difficult.
Secondly, and as a result of the heating, thermal gradients form in the laser medium, giving rise to the thermal focusing phenomenon resulting from differing indices of refraction in adjacent portions of the crystal at differing temperatures. This thermal focusing can result in damage to the laser medium. Thus it is always desireable to limit the thermal focal lengths in the laser medium to values much larger then the optical path through the medium, to avoid damage to the crystal structure. It is therefore desired to provide a laser having a high power output which avoids these problems.